Although it is applicable to any micromechanical components, the present invention and its underlying problem will be explained below with reference to acceleration and rotation rate sensors.
German Patent Application No. DE 195 37 814 A1 describes a method for manufacturing micromechanical sensors, for example acceleration and rotation rate sensors. Movable silicon structures, whose motions are sensed quantitatively by determining capacitance changes, are generated. The movable silicon structures are generated in an etching step, trenches being generated with a high aspect ratio in the silicon layer. In a second step a sacrificial layer, for example an oxide layer, is removed from beneath the micromechanical functional layer made of silicon. In a subsequent process the movable silicon structures thereby obtained are hermetically closed off, for example with a cap wafer that is applied using a seal-glass soldering process. Depending on the application, a gas atmosphere having a desired or suitable pressure is enclosed inside the volume closed off by the cap wafer.
In rotation rate sensors, a very low pressure is typically enclosed, for example on the order of 1 mbar. The background for this is that in these rotation rate sensors, a portion of the movable structure is driven resonantly. At very low pressure, it is very easy to excite a desired vibration with relatively small electrical voltages, because of the very low damping.
In acceleration sensors, on the other hand, it is usually not desirable for the sensor to vibrate, which is possible when a corresponding external acceleration is applied. These acceleration sensors are therefore operated at higher pressures, for example at 500 mbar. In addition, the surfaces of such acceleration sensors are often also equipped with organic coatings that prevent adhesive bonding of the movable structures.
If the intention is to manufacture very small and economical combinations of rotation rate and acceleration sensors, this can be done by integrating both a rotation rate sensor and an acceleration sensor onto one chip. The two sensors are manufactured simultaneously on one substrate. Such sensor combinations are encapsulated at the substrate level by way of a cap wafer that provides two separate cavities per chip.
The different pressures that are required in the cavities of the rotation rate sensor and of the acceleration sensor can be achieved, for example, by using a getter. For this, a suitable getter is locally incorporated into the cavity of the rotation rate sensor. A high pressure is initially enclosed in both cavities. The getter is then activated by way of a temperature step, whereupon said getter then pumps or getters the cavity volume of the rotation sensor to a low pressure.